Electronic substrates of both organic and ceramic construction typically have a plurality of conductive layers separated by the insulating material. Electrical connection is made between layers by means of plated through holes which in the case of organic insulators are usually plated along with plating a layer of conductive metal--e.g. copper on the top and bottom substrate surfaces. This plating is done prior to patterning or personalizing the surface conductive layers to form the interconnection geometry such as circuit lines, lands, and component or wire mounting pads. Other holes and openings may also be made in the substrate to accommodate mounting hardware or provide clearance for component parts to pass through the substrate. These other holes may be plated or unplated depending on the application.
Drilling and punching are two ways of forming the holes and apertures (hereinafter also referred to as holes) which are widely practiced in the electronic substrate industry, with drilling predominant in the manufacture of epoxy-glass substrates.
In the past, a large proportion of the plated through holes in a substrate provided both the interconnection function described above as well as accommodated a component lead of a pin-through-hole component. The component lead was then securely bonded mechanically and electrically to the plated walls of the hole by a soldering process such as wave solder or vapor phase soldering. However, with the shift to surface mount components, most plated through holes now perform only the interconnect function with no need to accommodate a component lead. Consequently, designers and manufacturers of electronic substrate have reduced the diameter of the holes as much as possible until a minimum drill diameter is reached below which drill breakage became excessive or below which plating fluids will not reliably enter the hole. Various advances in drill materials, drill designs, and drill processing are directed toward allowing smaller holes to be drilled and therefore reduce the proportion of valuable circuit real estate of a substrate that is needed for the through holes. However, drilled holes are currently still much larger than necessary merely to provide an electrical connection between layers.
It is also desirable to fill the holes after plating and planarize the surface so that additional layers of conductors and insulators can be positioned over the holes and patterned thus permitting a high wiring density on additional conductor layers by running circuit lines either partially or directly over the holes. Consequently, various techniques have been developed to permit such hole filling. With all hole filling techniques it is necessary to prevent the hole filling material from contacting the top and bottom surface of the substrate or to remove it prior to curing. Otherwise, the presence of hole filling material interferes with subsequent processing such as etching, plating, adhesion or lamination of the substrate surface. Thus, the filling of through holes in a printed circuit board is usually carried out by silk screen printing.
In U.S. Pat. No. 5,669,970 Balog et al. place a stencil having holes in a pattern corresponding to the holes in a substrate, in contact with the substrate. Solder paste material is then spread over the stencil where it passes through the holes in the stencil and into the holes in the substrate without contacting the surface of the substrate. The upper surface of the stencil has relieved portions to increase friction between upper surface and the solder paste thereby permitting increased speed of spreading with a squeegee blade.
Seki et al. in U.S. Pat. No. 5,277,929 adds an additional masking seal over some holes to prevent filler material from entering specific holes.
Gruber in U.S. Pat. No. 5,673,846 places a decal strip with holes aligned to holes in a mold over the mold. Liquid solder is then injected through the holes in the decal to fill the holes in the mold.
Hart et al. in U.S. Pat. No. 5,435,480 uses a stainless steel template on the top surface of a printed circuit card to control the flow of solder balls through a hole in the template and into a hole in the circuit card. An adhesive film such as polyimide is placed on the bottom side of the substrate to prevent the solder balls from falling out the bottom side of the substrate prior to reflow.
Masking techniques as described above require fabrication of a mask having holes arranged to correspond to the holes in a substrate which are desired to be filled. Accurate fabrication of such a mask is an expensive procedure. Furthermore the mask must also be accurately aligned to the substrate and held into contact with the substrate to prevent fill material from leaking between the mask and the substrate. Usually the mask must be removed from the substrate by peeling after the fill material has been applied and before cure of the fill material. The mask may be re-used, if not damaged during removal, and sometimes after cleaning of excess fill material, on another substrate having an identical hole pattern. Additional masks are needed for other hole patterns requiring an extensive library of masks to accommodate the various hole patterns encountered in substrate manufacture.
In accordance with the present invention there is provided a method of hole filling which eliminates the need to accurately fabricate, align, and maintain a library of hole filling masks.
It is believed this method represents a significant advancement in the art of substrate manufacture.